Want a more powerful electric shower? It's the cable that decides, not the shower

It's one of the most common questions I get, and it turns up on every DIY forum going: "My electric shower's a bit weak... can I just swap it for a more powerful one?" The honest answer is: maybe, but the shower is the easy part. Whether you can go from, say, an 8.5kW unit to a 10.5kW one comes down to the cable buried in your wall and the circuit feeding it... not the shiny new box you've got your eye on.

A bigger number pulls more current

Here's the bit the shower packaging doesn't dwell on. That kW rating is how much power the shower draws, and a more powerful shower pulls proportionally more current through the cable and the breaker.

Roughly speaking, an 8.5kW shower draws in the region of 37 amps. A 10.5kW draws closer to 46. That's a big jump, and every part of the circuit (the cable, the breaker in your consumer unit, the isolator switch, right back to the main fuse where the supply enters your home) has to be able to carry it safely. Fit a thirstier shower onto a circuit that was only ever sized for the old one, and you're asking the wiring to do more than it was ever designed for. That's where it gets dangerous.

Why an undersized cable is genuinely dangerous

When a cable carries more current than it's rated for, it heats up. And here's the part that catches people out: the breaker doesn't always save you. A breaker protects against a sudden fault or a dead short... it isn't a thermometer for a cable that's just quietly running too warm, day after day. An undersized or poorly routed cable can sit there cooking gently inside a wall or under the loft insulation, long before anything trips. Over time that degrades the insulation around the conductors, and in the worst cases it chars and becomes a real fire risk... hidden in a wall or ceiling where you'd never see it coming.

It's made worse by where the cable runs. A cable bundled up with others, or buried in thermal insulation in the loft, can't shed its heat, so it can safely carry less than the same cable would out in open air. That's why "what size cable" is never a number you can just read off a chart at the counter... it depends on the whole run, end to end.

What I check before I say yes

When someone asks me to fit a more powerful shower, this is the part that earns my fee, and it's why I won't quote it blind over the phone. Here's what I'm actually looking at:

• The existing cable... size and length. I check what's genuinely in the wall (not what someone assumed was there), how long the run is, and whether it can carry the new load. Length matters as much as thickness, because of voltage drop over a long run.

• How it's routed and insulated. Whether it's clipped in free air, buried in loft insulation, or bundled with other cables completely changes what it can safely carry. That's a calculation with correction factors, not a guess... and it's exactly why you shouldn't try to size and swap this yourself.

• The breaker and the RCD. The protective device has to match both the shower and the cable, and a shower circuit needs its own 30mA RCD protection.

• The main fuse and your supply. A hungry shower on top of a hob, oven and everything else can push an older 60A supply near its limit. Sometimes the house itself is the constraint, not the bathroom.

• The isolator and the bathroom rules. A shower needs a means of local isolation (usually a double-pole pull-cord in the room, or a switch outside the door) and everything has to respect the bathroom zones, because ordinary switches aren't allowed near the water.

• Earthing and bonding. I check the earthing arrangement is sound and that any bonding the bathroom needs is actually there.

Notice what's missing from that list: me telling you to go and buy "10mm cable" off the internet. The right cable for your shower depends on all of the above, and getting it wrong is the whole danger... so sizing it is my job, on site, not a table you follow at the trade counter.

The honest outcomes

When I've had a proper look, it usually lands in one of three places.

Sometimes it's fine. The existing cable and circuit have enough headroom, the routing's sensible, the supply can take it... and the swap really is close to like-for-like. Happy days, and I'll say so.

Sometimes it needs a new cable run. The old cable can't carry the new shower safely, so it needs replacing with a suitable one on the right protection. More work than a straight swap, but it's the difference between a shower that's safe for years and one that's quietly heating your wall cavity.

Occasionally the supply itself is the limit. Now and then the constraint isn't your wiring at all... it's the incoming supply or main fuse, which is owned by your network operator, not me. In that case, part of the answer is a conversation with them, and I'll tell you that honestly rather than cramming in a shower the supply can't really support.

Why I see this so often round here

I come across it a lot in the 1930s semis around Slough and Maidenhead. Lovely houses, but a good number still have their original shower circuits run in 6mm² cable that was perfectly fine for the modest shower fitted years ago. Then someone buys a 10.5kW unit expecting a five-minute swap, and the honest answer is that the cable needs upgrading first. It's not me being awkward or padding the job... it's that the old cable was never sized for the shower they've just carried home.

So before you order that more powerful shower, it's worth having the circuit checked... it'll tell you whether it's a simple swap or a bit more, before you've spent the money. I cover Slough, Windsor, Maidenhead and across the Thames Valley. If your current shower is tripping, smells hot, or the switch feels warm to the touch, don't wait... that's worth an emergency call-out today. Otherwise I can check the whole circuit properly as part of an inspection and test, or just get a free no-obligation quote and I'll tell you straight what your shower upgrade actually needs.